Electron discharge devices



ct. 2, 1956 G. H. ROBERTSON Er A1. 2,765,421

'ELECTRON DISCHARGE DEVICES Filed Feb. 1952 ATTOR/VFV nited StatesPatent 2,765,421 Patented Oct. 2, 1956 rice ELECTRN DISCHARGE DEVICESGeorge H. Robertson, Summit, and Edward J. Walsh, Morristown, N. J.,assignors to Bell Telephone La oratories, incorporated, New York, N. Y.,a corporation of New York Application February 8, 1952, Serial No.270,599 8 Claims. (Cl. S15-3.5)

This invention relates to electron discharge devices and moreparticularly to such devices known as traveling wave tubes wherein ahigh frequency signal is 'amplified by the interaction of theelectromagnetic wave associated therewith andthe electromagnetic waveassociated with an electron beam traveling adjacent thereto.

Such devices employ a helical conductor to guide the high frequency`signal withinthe device, the electron beam being projected along thehelical conductor. At the frequenciesl at which such devices areutilized, the high frequency signal may be introduced through a coaxialinput terminal land removed from the device, after amplification,through a 'coaxial output terminal. The high frequency signal from thecoaxial input terminal must therefore be coupled to the helix of thetraveling wave tube and then coupled to the coaxial output terminal.

An object of this invention is to-provide improved input and outputcoaxial terminalsV for a traveling wave tube. More specifically, it isan object of this invention to improve t-he coupling of the highfrequency signal to the helix of the traveling wave tube from coaxialinput and output terminals.

A further object of this invention is to mount 'and support the electrongun and electron collector assemblies from the coaxial input and outputterminals, respectively,

in an improved manner facilitating the fabrication of L traveling wavetubes employing coaxial input and output terminals.

These and other objects of. this invention are achieved in one specificillustrative embodiment of this invention wherein the envelope of atraveling wave tube comprises a metallic outer cylinder defining aportion of the envelope of the tube and end caps across each endthereof, the` caps having windows, e. g., vitreous beads, transparent tothe passage of microwave energy sealed therein. Wires extend coaxiallythrough the vitreous beads into the envelope thus defined. In accordancewith a feature of this invention, the wires define the inner conductorof the coaxial terminals and the envelope they outer conductor thereof.Metallic cup members are secured within the envelope to the interiorwall of the cylinder and have each a sleeve attached thereto to whichthe helix is electrically connected. The wires extend through the helixso as to project therein for a few turns of the helix.

In this specific embodiment, the electron gun comprises a cathode and anaccelerating electrode supported within one of the cup-shaped members,the cathode having an 'aperture centrally in the emissive surfacethereof through which the wire defining the inner conductor of thecoaxial input terminal extends. The aperture may be made of sufficientdiameter so that the wire does not contact the emissive surface or aninsulating button may be placed in the aperture, the wire extendingthrough the insulating button and being positioned thereby.

Further in this specific embodiment of this invention, the electroncollector is a cup-.shaped electrode supported by the wire extendingthrough the cap vitreous bead and dening the inner conductor of thecoaxial output terminal, the wire projecting axially within the helixfor a few turns thereof.

Thus, in this specific embodiment of the invention,A the innerconductors of the Vcoaxial input and output terminals extend within thehelix for a short distance, `the helix being electrically connected tothe outer conductors of the terminals. Each wire and the helix define agap in which the wire acts 'as a probe. As the wire extends past the endofthe sleeve to which the helix is connected, the gap has an electriclield of high intensity therein extending axially in the helix, enablingthe electromagnetic wave energy being transmitted along the coaxialterminals to be coupled to the transmission circuit defined bythe helixof the traveling Wave tube.

It is therefore one feature of this invention that the helix of thetraveling wave tube be electrically connected to the outer conductor ofthe coaxial terminal, the inner conductor of the terminal being a wireextending into the helix so that a gap of high field concentrationhaving a large axial Vcomponent is defined between the wire and the endof the outer conductor connected to the helix whereby coupling of thehigh frequency energy is attained between the helix and the terminal.

In accordance with a further feature of this invention, the outerconductor of the coaxial terminals is cup-shaped and has a sleeveportion to which the helix is electrically attached, the wire definingthe inner conductor of the helix extending through the sleeve portion.More specifically, in accordance with this feature or" this invention,the cath.- ode yand accelerating electrode are positioned withinV thecup-shaped outer conductor, the cathode having an aperture in itsemissive surface through which the wire delining the input innerconductor extends.

It is a further feature of this invention that the collector electrodebe supported by the wire defining the inner conductor of the outputterminal, 'and be electrically connected thereto so that maximumextraction of energy to the output circuit may be attained.

In accordance with a still further feature of this inf vention, thecoaxial terminals lare axially aligned with the helix Vof the travelingwave tube and define closure end .caps for the envelope thereof. Morespecifically, it is a 'feature of this invention that the envelope ofthe travel ing wave tube comprise a metallic cylinder and` closure endcaps having a central conductor therethrough, the closure end capsdefining the coaxial terminals of the vtraveling wave tube.

It is a further feature of this invention that the cupshaped membershaving sleeve portions to which the helix is connected are supported:and positioned by the outer cylindrical metal envelope to which theyare electrically connected, and the electrical distance from the sleeveportions to the coaxial terminals is determined by the positioning ofthe cup-shaped members in contact with the envelope, whereby thecup-shaped members may be positioned a predetermined electrical distancefrom the coaxial terminals to attain optimum impedance transformationbetween the helix and the external coaxial systems, the optimumelectrical distance being determined by the frequency or range offrequencies o-f operation.

A complete understanding of this invention and ofthe various featuresthereof may be gained from consideration of the following detaileddescription and the accompanying drawing, Fig. l of which is an externalView of a traveling wave tube illustrative of one specific embodiment ofthis vinvention and'Figs. 2 and 3 are sectional views of portions ofthetube of Fig. 1, Fig. 2 being a sectional view of the input terminal andelectron gun assembly and Fig. 3 being a sectional Vview of the outputterminal and electron collector assembly.

Referring now to the drawing, Vthe specific illustrative embodiment ofthis invention depicted comprises a metallic cylinder forming a portionof the envelope of the traveling wave tube. End closure caps 11 and 12are hermetically sealed, as by brazing, to the ends of the cylinder 10,each of the caps 11 and 12 having a nose portion v14, to which isthreadedly engaged the outer conductor 15 of a coaxial connecting jackor plug 16. A glass or ceramic bead 17 is advantageously sealed wtihinthe nose portion 14 to complete the vacuum tight envelope of thetraveling wave tube. A thin wire 18 is sealed axially within the glassor ceramic bead 17 of end closure cap 11 and extends into a cap 19 towhich it is electrically secured, as by solder 20. Spring ngers 22 atthe end of the inner conductor 23 of the coaxial connecting jack 16encompass the cap 19. Similarly a wire 24, advantageously considerablylarger than wire 18, extends axially through the glass bead 17 of endclosure cap 12 and is sealed thereto, the spring fingers 22 of the innerconductor 23 of the coaxial connecting jack 16 similarly encompassingthe end of the wire 24.

Adjacent the end closure cap 11 is a cup-shaped support member 25secured, as by brazing or welding, to the outer cylindrical envelope 10.The base of the support member 2S is apertured and a sleeve 26,advantageously integral therewith, is dependent therefrom. Positionedand supported within the cup-shaped support member 25 is the electrongun assembly 28 which may advantageously comprise a cathode 29 and anaccelerating electrode 30. The cathode 29 comprises a disc 32 having asmall aperture 33 axially therein and an electron emissive coatingthereon, a heater cup 34 across the end of which the disc 32 extends, aheater element 35 positioned within the heater cup 34, an outercylindrical heat shield 36 having flanged portion 38 advantageously inthe plane of the 'disc 32, a base member 39 secured, as by weldingwithin the base of the cylinder 36 at the opposite end from the flangedportion 38, and a plurality of support strips 40 supporting the heatercup 34 from the base member 39. An insulator washer 42 is positioned onthe base of the support member 25. The accelerating electrode is betweenthe insulator 42 and an insulator washer 43 while the anged portion 38of the cathode cylinder 36 is between the insulator 43 and a thirdinsulator washer 44. A plurality of springs 45 advantageously welded tothe inner wall of the envelope cylinder 10 bears against the insulatorsand sandwiched accelerating electrode and flange. Insulators 42 and 44are advantageously slightly dished to receive the accelerating grid 30and anged portion 38 to prevent lateral motion of these members.

The helix 47 of the traveling wave tube is secured, as by being glazedthereto, to a plurality, as three, of insulating rods 48 which in turnare secured, as by glazing, to the inner surface of the sleeve 26 ofcup-shaped support member 25. The one end of the helix 47 iselectrically connected to the sleeve 26, as at 49. The other end of theinsulating helix support rods 48 is secured, as by brazing, to the innersurface of a sleeve 51 of a cup-shaped member 52 to which the other endof the helix 47 is electrically connected, as at 53. The cup-shapedmember 52 is similar to the cup-shaped member 25 but is advantageouslysecured to the envelope cylinder 10 by a plurality 'of spring fingers 55rather than by being brazed or welded thereto to allow for the differentthermal expansions of the envelope 10 and the insulating rods 48. Acup-shaped electron collector S7 is supported by the wire 24 within thecup-shaped member 52.

Direct-current connections are made to the heater element 35, cathode 29and accelerating electrode 30 by individual leads 59 connected to eachof these elements and to terminal pins 60 extending through a glass orceramic bead 61 in the envelope cylinder 10. A high frequency filter 63is connected between the terminal pins 60 and the envelope cylinder 10external to the envelope to prevent passage of radio frequency power tothe directcur rent supplies and comprises a shell 65 positioned with itsbase on the envelope cylinder 10, a pair of button capacitances 66electrically connected between the terminal pin 60 and the shell 65 anda choke 67 between the capacitances 66 and comprising a few turns ofwire.

Getters 70 may advantageously be connected between the terminal pins 60and the inner wall of the envelope 10 and burned out during theprocessing of the device. An exhaust tubulation 71 is advantageouslyprovided in the end closure cap 11.

In the operation of traveling wave tubes in accordance with thisspecific embodiment of this invention, the electromagnetic energy isguided by the external wave guide or coaxial line system to the coaxialjack 16 and thence to the coaxial input terminal comprising the closurecap 11 and the wire 18. The wire 18, which denes the inner conductor ofthe coaxial input terminal, extends through the cathode 29 andspecifically through the heater element 35 and the aperture 33 in thecathode disc 32 and axially within the helix 47 for a short distance,advantageously extending past the end of the sleeve 26 a short distance.VAn insulating coating may be advantageously applied to the centralportion of the wire 18 where it extends through the heater element 35and the aperture 33 in the cathode disc 32. Also if desired formechanical support, an insulator button may be positioned Within theaperture 33 in the cathode disc 32 to aid in accurately aligning thewire 18.

The outer conductor of the coaxial input terminal is defined by theclosure cap, the cylinder 10, the cup-shaped support member 25 and thesleeve 26 thereof. The sleeve 26 may extend along the rods 48 so as toencompas a few turns of the helix 47, the helix being electricallyconnected to the sleeve 26. The wire 18 and the helix 47 thus define agap in which the wire 18 acts as a probe, the gap having an electric eldof high intensity therein with a large axial component whereby theelectromagnetic energy is coupled from the coaxial system to thetransmission circuit delined by the helix 47.

Similarly, the output coaxial terminal is defined by nose portion 14 ofthe end closure cap 12 and the wire 24. The outer conductor of thecoaxial output terminal is thus the closure cap 12, the envelope 10, thecup-shaped member 52 and sleeve 51 thereof. The helix 47 may extendwithin the sleeve 51, and the wire 24 extends a short distance past theend of the encompassing sleeve 51. Coupling to the output coaxialterminal is attained by the high eld developed around the end of thewire 24 and the portion of the helix adjacent thereto. As the electronstream projected by the electron gun assembly 28 is modulated by theelectromagnetic wave energy guided by the helix and the electron streamimpinges on the collector 57 supported by the wire 24, radio frequencyenergy may be extracted from the electron beam as well as from theelectromagnetic wave guided by the helix.

Advantageously, in the operation of this embodiment of this invention,the envelope 10 and thus the helix 47 are at ground potential. Thecathode 32 is maintained negative with respect to the helix while theaccelerating grid 30 will be positive with respect to the cathode 32.The collector 57 is also advantageously maintained positive .withrespect to the helix 47 so that secondary electrons will be connedwithin the cup of the collector and not impinge on the helix. However,the collector may advantageously be operated at the same direct-currentpotential as the helix or at a slightly lower potential if the secondaryemission problem is not important. The particular values of voltagesemployed would depend on the frequency range of operation as thefrequency is determined in part by the potential difference between thehelix and the cathode. The particular values of voltage would alsodepend on the current density, the desired gain and the particularoutput power and other operational criteria.

The electrical length of the path from the coaxial input terminaldefined by the nose portion 14 to the gap defined by the wire 18 and thesleeve 26 to which the helix 47 is connected can be varied to obtain theoptimum impedance match between the external system and the helix. Inthis specio embodiment of this invention this may facilely be doneduring the `iabrication of the device by varying the position along theenvelope l() at which the cup-shaped member 25 is secured thereto, as byWelding or brazing. The point at which the cup-shaped member 25 will besecured to the envelope 10 for optimum imedance transformationl willdepend on the desired frequency, or frequency range, of operation.Similarly optimum impedance transformation between the helix 47 and theoutput coaxial system can be obtained by positioning the cup-shapedmember 52 with the envelope 10, by the spring fingers 55, at the optimumelectrical distance from the output nose portion 14 for the frequency ofoperation.

In the fabrication of the specific illustrative embodiment of thisinvention depicted in the drawing and described above, the helix d'7,helix support rods 48 and cup-shaped members 25 and 52 areadvantageously assembled first as a unit and then slid into the cylinderlil and the cup-shaped member 25 secured, as by welding, to the innerwall of the cylinder. The closure cap 12 in which has priorly beensealed the window ll7 is then hermetically positioned across the end ofthecylinder l0, as by being brazed thereto. The insulator washers 42, i3and 44, accelerating electrode and priorly assembled cathode assembly 28are positioned with the cup-shaped member 25, being automaticallycentered thereby and by the disced portions of the washers 42 and 44.The spring ngers are then attached to the inner wall of the cylinderit?, as by welding, and bias the sandwiched washers and electrodes inposition. After the leads 59 have been attached to the terminal pins 6dand the getters positioned across the pins 6b and the envelope 10, theend closure cap Trl is hermetically sealed across the end of thecylinder 1th, as by being brazed thereto. The wire 18 is priorly sealedin the window i7 in the end closure cap l1 and extends through theaperture 33 in the cathode surface 32. when the end closure cap l1 is inplace. The envelope is exhausted through the tubulation 71, which maythen be sealed oi'.

Reference is made to application Serial No. 270,598, filed February 8,1952, by G. H. Robertson and application Serial No. 270,721, filedFebruary 8, 1952, by I. R. Pierce wherein are described relatedinventions.

lt is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

l. A traveling wave tube comprising an envelope, a helix extendingwithin said envelope, electron gun means for projecting a stream ofelectrons along said helix, said gun means including a cathode having anemissive surface and an aperture in said surface, and a coaxial inputterminal defining a portion of said envelope, said terminal comprising awire inner conductor extending through said aperture and projectingaxially into said helix a short distance and a cylindrical outerconductor having a portion through which said helix extends and to whichsaid helix is electrically connected, the ends of said inner conductorand said outer conductor portion dening a gap having an electric fieldof high intensity for coupling wave energy from said coaxial inputterminal to said helix.

2. A traveling wave tube comprising an envelope, a helix extendingwithin said envelope, electron gun means for projecting a stream ofelectrons along said helix, said gun means including a cathode having anemissive surface on one side thereof, an aperture in said surface, andheater means adjacent the other side of said cathode, a coaxial inputterminal defining a portion of said envelope comprising a wire innerconductor extending through said heater means and through said apertureand projecting 6 axially within said helix a short distance and acylindrical outer conductor to which said helix is electricallyconnected, said cylindrical outer conductor having a cupshaped portion,and means positioning said electron gun means in said cup-shapedportion.

3. An electron discharge device comprising an envelope, a helixextending Within said envelope, electron gun means for projecting anelectron stream along said helix, a coaxial output terminal, saidterminal comprising an inner conductor projecting into said helix and anouter conductor encompassing said inner conductor and electricallyconnected to said helix, the end of said inner conductor defining withsaid helix a high electric eld intensity gap for coupling wave energyfrom said helix to said coaxial output terminal, and a collectorelectrode supported by said inner conductor in the path of said electronstream.

4. An electron discharge device comprising an envelope and a helixextending within said envelope, said envelope comprising a cylindricalmember and a coaxial terminal across each end of said cylindricalmember, said coaxial terminals each comprising an outer conductor towhich said helix is electrically connected and a Wire inner conductorprojecting axially a short distance Within said helix past the ends ofsaid outer conductor, the ends of said wire inner conductor and saidhelix defining gaps having high electric field intensities for thecoupling of Wave energy between said coaxial terminals and said helix.

5. An electron discharge device in accordance with claim 4 wherein saidcylindrical member is of a conducting material and detines a portion ofthe outer conductors of each of said coaxial terminals.

6. A traveling wave tube comprising an envelope, a helix within saidenvelope, electron gun means for projecting a stream of electrons alongsaid helix, said gun means comprising a cathode including a memberhaving an emissive surface on one side thereof, an aperture in saidmember, and a heater adjacent said member to the other side thereof, acoaxial input terminal defining a closure for one end of said envelopeand comprising a wire inner conductor extending through said heatermeans and said aperture and projecting axially a short distance withinsaid helix and an outer conductor to which said helix is electricallyconnected, said outer conductor including a cylindrical cup-shapedportion, means positioning said electron gun means in said cup-shapedportion, a coaxial output terminal defining a closure for the other endof said envelope and comprising a wire inner conductor projectingaxially a short distance into said helix and a cylindrical outerconductor encompassing said inner conductor and electrically connectedto said helix and a collector electrode supported by said innerconductor in the path of said electron stream.

7. A traveling wave tube comprising an envelope, said envelopecomprising a cylindrical member and a coaxial terminal across each endof said member, a helix extending in said envelope, the axis of saidhelix being coincident with the axis of said envelope, electron gunmeans for projecting a stream of electrons along said helix, saidelectron gun means comprising a cathode having an emissive surface ouone side, an aperture through said surface, and a heater elementadjacent said cathode to the other side thereof, one of said coaxialterminals comprising a wire inner conductor extending through saidheater element and said aperture and projecting axially a short distanceinto said helix and a cylindrical outer conductor encompassing saidinner conductor and including a cupshaped portion to which said helix iselectrically connected, means positioning said electro-n gun means insaid cup-shaped portion whereby said electron gun means is supported andaligned by said cup-shaped portion, the other of said coaxial terminalscomprising an inner wire conductor projecting axially a short distanceinto said helix and an outer cylindrical conductor encompassing saidinner conductor and including a cup-shaped portion 7 to which said helixis electrically connected, means for References Cited in the le of thispatent supporting said helix from said cup-shaped portions of UNITEDSTATES PATENTS said terminals, and a collector electrode supported inthe v path of said electron stream by said inner conductor of 2427752StrenPel et al' Sept 23 1947 2,472,204 Fubini et al. June 7, 1949 saidother coaxial terminal. 5 q

8. A traveling wave tube in accordance with claim 7 2526399 qkress etal' Oct' 17 19'0 wherein said envelope cylindrical member is of a con-2578434 Lmdenblad Dec 11 1951 ducting material and defines a portion ofthe outer cou- 2611832 Lapostoue Sept' 23 1952 ductors of each of saidcoaxial terminals, said cup-shaped Eglyfn Este portions beingelectrically connected to and positioned 10 2,667,597 Bailey Jan' 26]1954 by said envelope.

OTHER REFERENCES Article by A. V. Hollenberg: Bell System Technical 15Journal, January 1949, pp. 52-58.

